1. Field of the Invention
The present application relates generally to building panels, and more particularly, to trough-edge building panels useful in constructing panelized wall systems with joints that are resistant to cracking.
2. Description of the Related Art
Every building tradition in the history of mankind has produced stuccowork. Examples of stuccowork range from the Aztec architecture of ancient Mexico to the architecture of North Africa and Spain. In modem times, stuccowork has been popular in residential construction since the 1920s, especially in dry, warm climates like the U.S. Southwest. Because of the many ways in which it can be treated, stucco remains a popular exterior finish for many building types. Since stucco is applied as a paste, it can be textured and will conform to almost any shape resulting in a smooth, seamless wall of monolithic appearance and sound structural integrity.
In spite of its ongoing popularity, many builders resist using stucco as an exterior finish in framed construction because of the problems associated with applying stucco to exterior walls. Traditionally, a stucco coating is a thin paste composed of Portland cement, sand, lime, and water. Successive layers of stucco paste are applied to a metal or plastic mesh fastened to the exterior of the wall. Stucco supported on framed construction is normally ⅞″ thick and is applied in three applications: the first or scratch coat, the second or brown coat, and the third, a finish, colored, texture coating. Since each layer of stucco paste must dry and harden before the next is applied, it takes several days to finish a traditional stucco wall.
While hundreds of thousands of new housing units are built every year, only a fraction of those units use stucco as an exterior finish. Stucco's susceptibility to moisture damage, for example, limits its use in wet climates. Likewise, stresses caused by transporting stucco-finished transportable buildings prevent its use in the lucrative manufactured housing market.
Another method of producing a textured or stucco look is a Direct-applied Exterior Finish System or DEFS. In DEFS, panels of a substrate material are fastened to the framing followed by a finish texture coating. The texture coating may be applied as a single coat or in multiple thin coats, and often uses either a joint or full-wrap alkali-resistant fiberglass mesh to reinforce the coating against cracking. DEFS can be installed and finished in a much shorter time than traditional stucco, enabling shorter construction times.
DEFS have not enjoyed a large share of the exterior market, however, because thin DEFS coatings, which are relatively brittle, are incompatible with the movements of wall panels. The substrate panels will invariably move with respect to each other from building settling, temperature variations, or moisture absorption. These movements can cause cracking of the finish at the panel joints. To prevent this cracking, the joints are often covered with tape or filled with caulk. In many installations, both tape and caulk are used. In DEFS stucco applications over fiber cement, an alkali-resistant fiberglass mesh tape, 2″ to 12″ wide, joins the two adjacent fiber cement panels that are the substrate over which the stucco is applied.
One problem with these types of joints is “joint read,” a phenomenon in which the joint underlying the finish is visible. Joint read breaks the desired monolithic appearance of the finished wall. Joint read is a particular problem with fiber cement substrates because fiber-cement-panel faces absorb moisture from the finish coat faster than the taped joint. This differential moisture absorption makes the joint visible. In joints covered with tape, the step formed by the edge of the tape and the panel surface can often be seen, especially with low-angle illumination. Cracks arising from the loss of adhesion or slipping at the edges of the tape cause another type of joint read.
“Peaking” is another type of joint read and is caused by movement of the fiber cement panels. This movement causes the adhesive bond between the joint tape and the caulk joint to fail, causing the stucco to separate from the caulk. As a result, the stucco covering the joint floats higher or lower than the surrounding area, giving the appearance of a peak. Peaking also results from the caulk shrinking during curing, pulling the adhered joint tape below the surface of the stucco. Peaking disrupts the monolithic appearance of the wall and destroys the integrity of both the stucco and the substrate.
Another option is a thin joint sealing tape. These tapes, however, are often waterproof. Consequently, they do not absorb the stucco mix, resulting in poor adhesion between the stucco and the tape, which leads to surface deformation.
A more serious problem is cracking at the joints. Cracking not only disrupts the monolithic look of the finish, but also allows moisture to get behind the stucco and rot or corrode the wood or steel structural framing. Furthermore, these cracks are entry points for insects or fungi, which can damage the interior of the wall.
Consequently, DEFS are rarely used where there is wide daily temperature variation especially when coupled with a high rate of wet/dry cycling. DEFS are also seldom used in the fast-growing manufactured housing market because of the additional stress placed on walls during transport. Solving the joint read and stucco-cracking problems could significantly expand the market for DEFS stucco applications using fiber cement and other substrates. Not only could builders use fiber cement substrates and stucco in wetter climates, but also fiber cement substrates and stucco use could be expanded to new markets such as manufactured housing and modular buildings.
One strategy for preventing cracking of a DEFS coating at the panel joints is to construct joints from elastomeric materials. These elastomeric joints absorb the stress created by the differential panel movements. Such joints may be used with flexible, latex-based texture coatings, often called latex stucco or synthetic stucco. These finishes are able to move with the joint without cracking, which would greatly expand the market for DEFS applications.
The effectiveness of such joints may be evaluated in test walls consisting of several panels assembled on a frame, constructed with the joint-to-be-tested. The test wall is finished with a DEFS coating and subjected to a racking test. The racking test applies an in-plane shear force to the test wall, resulting in relative panel movement, until the DEFS coating cracks. The distance of maximum deflection at which the finish cracks is a measure of performance of the panel joints. For example, in order to pass the International Congress of Building Officials (ICBO) AC 59 “Acceptance Criteria for Direct-Applied Finishing Systems” (September 1992), a test wall constructed according to the method described in ASTM E-72 (98) subjected to a racking load that causes the wall to deflect 1″, does not develop any visible joint cracks.
The polymeric adhesives used,in joint tapes in DEFS tend to soften and lose holding power at 120° F., a temperature often achieved on the exterior vertical walls of a building exposed to full summer sun. Fiber cement building panels may become saturated with water in wet environments if installed improperly or if the finish layer fails. Many adhesives bond poorly to wet substrates. The performance of adhesives used in manufacturing joint tapes for DEFS applications may be evaluated using the “180° peel test,” a well-known method of evaluating the adhesive strengths of tapes. The 180° peel test measures the force required to break the adhesion of a joint tape applied to two substrates held at a 180° angle.
U.S. Pat. No. 5,732,520 describes a method for forming a single-coat, synthetic-stucco-finished exterior wall. First, fiber cement wallboard panels are installed onto a building frame with the adjacent edges of the panels forming narrow gaps. Polyurethane caulk is applied to the gaps, and low-profile fabric-backed joint tape is applied over the adjacent edges of the panels to cover the gaps and the caulk. A high build flexible resinous latex emulsion in next applied directly over the panels and adhesive tape to form a synthetic stucco finish. The moisture absorption properties of the fabric from which the tape is manufactured matches that of the wall panels. A stucco-finished joint constructed according to this patent with 3″-wide joint tape slips and cracks at the edges when stretched 3–5 mm. The relative motion of adjacent 4′×8′ cementitious boards under normal conditions is greater than 3–5 mm, however. While the joint tape described in U.S. Pat. No. 5,732,520 distributes the joint movement somewhat, the adhesive used in this tape is not sufficiently strong to prevent the edges of tape from slipping under stress. Consequently, the edges of the tape slip, cracking the stucco coating. A wider tape, for example, a 6″-wide tape, might better withstand the movement, but at an increased cost.
Cracking may be also prevented by applying additional layers of stucco or a joint compound over the tape before applying the final coat of stucco. This method, however, is expensive, time consuming, and requires skilled workers. Moreover, this technique often fails to produce satisfactory results. Another method of preventing cracks is increasing the thickness of the stucco build. This method is also expensive and time consuming, however.